The NMR technique is based upon the magnetic properties of nuclei containing odd numbers of protons and neutrons. These nuclei possess an angular momentum related to the charge thereof. The magnetic moment is directed along the spin axis of each nucleus. When placed in a strong and generally homogeneous static magnetic field, designated B.sub.o, the nuclei either align with or against the applied field and precess with a common sense about the applied field. The precessional angle of a nucleus may be changed by absorption of elecromagnetic energy through a phenomenon known as nuclear magnetic resonance, NMR, which involves impressing upon the nuclei a second rotating magnetic field, designated B.sub.1, of frequency to match that of their normal precession. When the applied RF magnetic field is removed, the nuclei precess and relax toward their equilibrium conditions, generating radio frequency signals characteristic of the molecular environments in which the nuclei reside. The frequency at which they precess is known as the Larmor frequency and is given in annular frequency by .omega.=.gamma.B. .gamma., the gyromagnetic ratio, is a constant for each nucleus or nuclear isotope and generally results in widely separated Larmor frequencies for a given applied field strength, B.sub.0. B is the magnetic field acting on the nuclei and is modified by the molecular environment of a nucleus according to B=B.sub.0 (1-6). .delta. is the chemical shift offset impressed upon chemically equivalent nuclei by the local electronic distribution. Measured usually in parts per million, chemical shifts of a particular nucleus or nuclear isotope produce much smaller differences in frequency, and spectra derived from them can be used to obtain quantitative, structural, and dynamic information about the molecules of a sample. In another configuration, a bias or gradient in the normally homogeneous B.sub.0 field is introduced across the sample for the purpose of spatially encoding information into the NMR signals. Images are later reconstructed from the information contained within this data, forming the basis of NMR imaging, a technique now widely used in medical diagnostics. Because the Larmor frequency is proportional to the applied field B.sub.0 , local resonance frequencies will vary across the sample according to the strength of the magnetic field. It is only with technical difficulty that homogeneous B.sub.0 fields are obtained, and high-field magnets are usually provided with electronic shim coils to counter both residual distortions of the magnet and the susceptibility distortions from sample or tissue being investigated and from materials comprising NMR probe. Acquisition of highly resolved spectra from a sample is usually preceded by a "shimming" procedure using a high sensitivity NMR signal from hydrogen protons or another abundant nucleus. In very high-field spectroscopy, the stability of the magnetic field is further maintained by electronically comparing the frequency of an NMR signal derived from a separate nucleus, typically deuterium, with that of a stable RF oscillator and engaging a field sweep coil in the feed-back loop of the field stabilization circuit.
The B.sub.1 field for transmitting to the sample is derived most efficiently from a resonant radio frequency (RF) coil placed in proximity to the sample and connected to the RF transmitting apparatus. Either the same or a second RF coil may be connected to the RF receiving apparatus to receive the NMR signals, which are induced in the coil by the precessing magnetism of the nuclei. Free induction signals from chemically shifted nuclei and from samples with field gradients impressed upon them are normally received with a single-resonant coil tuned to the Larmor frequency of the nucleus. Transmitting to and receiving NMR signals from two different nuclei or nuclear isotopes, however, generally require use of two coils each single tuned or a single coil doubly tuned to the individual Larmor frequencies. The lossy circuit elements in double-tuned, single coil probes necessarily make them less sensitive than their single-tuned counterparts. Considerable care must be exercised to maintain maximal sensitivity at each frequency, and maximal sensitivity at one frequency is frequently achieved at the cost of less sensitivity at the other. Improved sensitivity and a reduction in transmitter power can be obtained if a coil can be operated in circularly polarized mode. See C.-N Chen, D. I. Hoult, and V. J. Sank, J. Magn. Reson. 54, 324-327 (1983). A linear oscillating field, such as produced by a simple resonant coil, can be cast as the sum of two circularly polarized components of equal amplitude. Likewise, by combining the linearly oscillating fields of two well-isolated single-tuned coils or the well-isolated fundamental modes of a multi-modal structure such as the "birdcage" coil (see Hayes et al., J. Magn. Reson., 63, 622-628 (1985)), a single, circularly polarized magnetic field can be produced which matches the precessional motion of the nuclei. Circularly polarized coils are similar to crossed-coil double-tuned probes in that two resonant circuits require tuning. They differ, however, in that being of the same frequency, they require a high degree of electrical isolation to operate independently, as will be discussed later.
Many clinical applications of NMR spectroscopy find their origins in conventional high-field chemical NMR spectroscopy. Dual and triple tuned probes are employed wherein one channel, tuned typically to deuterium, is used for shimming and field stabilization of the magnet. The NMR signal is derived from a small amount of typically deuterated solvent added to the sample. In another application involving acquisition of spectra from "X-nuclei," that is, nuclei other than protons having lower frequencies, lower abundances, and consequently lower sensitivity, the sensitivity of these nuclei can be enhanced via the Nuclear Overhauser Effect (NOE) and decoupling of proton spins by irradiating the sample with a modulated signal at the proton frequency. The NOE increases the polarization of X-nuclei by transferring it from the proton spin population, which by virtue of its higher frequency exhibits greater polarization. Finally, heteronuclear experiments are employed wherein RF pulses are applied simultaneously or at alternating intervals to the populations of two nuclei to derive chemical bond and other information related to spins that are proximate to one another. One class of these experiments termed "indirect" experiments, relies upon the greater sensitivity of the protons signal to yield structural and dynamic information about much lower abundant and less sensitive nuclei.
In performing medical NMR spectroscopy, the NMR instrument is generally configured to observe a single nucleus such as hydrogen protons (1H), phosphorus-31 (31P), or carbon-13 (13C). Since phosphorus containing metabolites are key indicators of the state of tissue, considerable effort has been directed towards acquiring and analyzing phosphorus spectra from tissue. Acquisition of high sensitivity phosphorus and other spectra from human tissue is presently being investigated as a technique for identifying and characterizing tissues and following their response to treatment. For the human brain, in particular, a "birdcage" coil of reduced axial length for improved sensitivity has been developed (see U.S. Pat. No. 4,885,539). The coil, owing to its reduced length, provides an improvement in circuit Q by reducing RF eddy current losses in normally highly conductive tissue. The patient examination is complicated, however, first by the need to shim the static magnetic field, B.sub.0, using the hydrogen proton resonance from tissue and, second, the need to acquire proton images of the patient for later correlation of spectral data with the patient anatomy. Typically, a separate coil tuned to the proton frequency is employed for shimming and imaging, and is subsequently replaced with the phosphorus coil for acquisition of in vivo spectra. It is apparent that removal of the patient from the magnet during the exam is unnecessary with a coil which is dual tuned for both frequencies patient examination time is thereby reduced. An additional benefit of a dual tuned resonator is the ability to perform proton decoupling by transmitting a suitable decoupling waveform at the second, proton frequency. Proton decoupling improves the resolution and sensitivity of X-nuclei by averaging and effectively removing the coupling between the different nuclear spins. Decoupling efficiency increases with increased transmitted power. At a B.sub.0 field of 1.5 Tesla, however, the power deposition from head coils operating in linear mode approaches acceptable limits set forth in FDA guidelines. Power deposition will increase with B.sub.0 field, owing to the greater losses in tissue at higher frequency. It is therefore apparent that operation of the proton channel of a dual tuned resonator in circularly polarized mode can reduce power deposition and/or improve decoupling efficiency.
Implementation of a dual tuned resonator operating simultaneously in circularly polarized (CP) mode at both frequencies has not been possible using existing methods. In attempting to implement dual tuned CP volume resonators, reactive elements have been incorporated into the conductors of normally single-tuned birdcage coils to make them doubly resonant. See A. R. Rath, J. Magn. Reson., 86, 488-495 (1990). With capacitors chosen to be less lossy than the inductors of the coil, the low frequency mode suffers a greater loss in efficiency when compared with the performance of its single-tuned counterpart. It is a further feature of this method of double tuning that additional inductors do not contribute substantially to the generation of the RF magnetic field, B.sub.1, in the sample at either frequency. Because of the strong interaction between tuning elements, these resonators exhibit complex tuning, matching and mode alignment problems at both NMR frequencies. The process of tuning, matching and mode alignment at both frequencies is an iterative one with iteration not necessarily leading toward the optimal outcome. Thus, true simultaneous operation of these dual tuned resonators circularly polarized mode has not yet been possible.